The 155 falls into a category referred to as "upholstery class" so should be more than adequate for what you want to do. For holsters you need a heavy-duty machine due to the thickness of leather involved. The most commonly used ones are referred to as class 4 machines, clones of a Juki 441. These have long thick needles, will handle thicker thread and clear 3/4" - 7/8" under the foot (most upholstery class clear 3/8", I think the 155 will clear 1/2").
One other thing to consider, it's likely the machine has a clutch motor and these can require some time to master if you want to sew slowly (which most of us do when working with leather). Replacing the motor with a servo motor, and possibly a speed reducer too, will make slow speed sewing much easier and may save you much frustration.